The goal of this proposal is to study mechanisms by which non-tumor cells within the tumor microenvironment (TME) promote the development and progression of esophageal cancer. This proposal will test the hypothesis that GR1+CD11b+ myeloid derived suppressor cells (MDSCs) play an essential role in promoting esophageal cancer through activation of stromal fibroblasts. Previous work has demonstrated the capacity of MDSCs to activate fibroblasts. The first aim of this proposal is to elucidate the mechanism whereby tumor-conditioned MDSCs activate fibroblasts and characterize the role of this activity on tumor progression. Co-culture experiments will be used to determine whether the capacity to activate fibroblasts is a characteristic of all MDSCs or a unique property of MDSCs conditioned by tumor-derived factors. Activation of quiescent fibroblasts will be determined using immunohistochemistry (IHC) for a-SMA and Fsp-1, established markers of fibroblast activation. Cytokine array analyses of conditioned media from the MDSC-fibroblasts co-cultures will allow investigation into the mechanism of MDSC-mediated activation of fibroblasts. Candidate factors will be evaluated using neutralizing antibodies and RNAi. Finally, a panel of fibroblasts activated in vitro with recombinant TGF-b, recombinant IL-6, or conditioned media from various sources, including MDSCs and esophageal tumor cell lines will be co-injected with esophageal tumor cells into nude mice to determine whether the source of fibroblast activation influences the capacity to promote tumorigenesis. The second aim of this study will be to investigate the requirement of MDSCs initiation and progression of esophageal tumors. This aim will be undertaken using Histidine Decarboxylase-null (HDC-/-) mice, which exhibit elevated MDSC activity, resulting in increased tumorigenicity when exposed to carcinogens. L2-Cre;p120flox/flox mice serve as a model of esophageal cancer. HDC-/- mice will be used to generate HDC-/-;L2-Cre;p120flox/flox mice to study the effect of enhanced MDSC activity on the tumor phenotype in this mouse model of esophageal cancer. In a complementary approach, we will generate transgenic mice expressing HDC under the control of the CD11b promoter (CD11b-HDC). MDSCs in these mice will maintain expression of HDC in the tumor microenvironment, promoting differentiation and preventing the accumulation of the immune suppressive cells in the tumor stroma. Transgenic HDC mice will be used to generate CD11b-HDC;L2-Cre;p120flox/flox mice to determine the impact of MDSC inhibition on esophageal cancer initiation and progression with particular attention paid to the effect on activated fibroblasts in the TME. Esophagi, forestomachs, and oral tissues containing tumors will be fixed in buffered formalin, and histological analyses performed. Furthermore, IHC for a-SMA and Fsp-1 will be employed to determine the activation status of cancer-associated fibroblasts (CAFs). In aggregate, fulfillment of the aims described above will provide insight into mechanisms of activation of CAFs by MDSCs, and determine whether MDSCs are required for intiation and progression of esophageal cancer.